Monitoring Ocean Concentrations of Aromatic Hydrocarbons from Produced Formation Water Discharges to Bass Strait, Australia

Abstract

Abstract Previous assessment of dispersion modelling, toxicity testing, and characterisation of produced formation water (PFW) discharges into Bass Strait indicated a very low environmental risk from PFW to the marine environment. Peak PFW concentrations can exceed the effect levels (EC50 or LC50) measured in 24-96 hr laboratory toxicity tests only within distances of tens of metres from the discharge point. In this assessment, the field monitoring of aromatic hydrocarbons in the water column (which are in low concentrations in PFW) was undertaken to directly assess dispersion and predicted fate mechanisms. Very low concentrations of both light and heavier aromatic hydrocarbons are likely in any PFW discharge. A high volume absorption sampler was deployed 20 m from the discharge point to continuously sample ocean concentrations of aromatic hydrocarbons for up to one week, providing large volume (1,000 L) water samples. Gas Chromatography/Mass Spectrometry (GCMS) was used to measure aromatic hydrocarbon concentrations. The concentrations measured with the ocean sampling device provide time integrated samples over approximately one week, and the results showed that the ratio of discharge concentration to ocean concentration was approximately 20,000:1. Compared to dispersion modelling predictions, the ocean sampler indicates lower environmental risk. This is because dispersion modelling predicts ocean concentrations within the plume whereas the sampler is measuring concentrations at a fixed point over the long term and is exposed to the plume only intermittently, similar to a sessile marine organism. Therefore the ocean concentrations provided by the large volume sampler are more representative of longer term ocean concentrations which can be experienced by marine organisms. Further assessment of prevailing operational and oceanographic conditions in Bass Strait suggests that there does not appear to be a water column accumulation of PFW aromatic hydrocarbons adjacent to the discharge. Introduction There are currently 12 offshore produced formation water (PFW) discharges from Esso/BHPP's oil and gas production facilities in Bass Strait, Australia (Figure 1). Total volumes of the discharges are approximately 90 ML.d-1. The volumes have increased significantly in the past five years from the minimal discharges in the early 1970s when oil production was brought on-line starting with Halibut in 1970. PFW discharge volumes are likely to increase further as fields mature. Environmental risk assessment of PFW discharged into Bass Strait suggests the discharges present a low potential for impact on marine organisms due to low acute toxicity and high dilution rates. Terrens and Tait reported discharge plume dispersion modelling predictions where the dilution of a typical PFW discharge in Bass Strait was 30:1 within 10 m of the discharge point, and that acceptable acute toxicity could be obtained with dilution of less than 4:1. The dispersing discharge plume is spatially limited as a narrow band, and mobile due to local tidal conditions. The length of any water column organism's exposure to concentrations in excess of acute effect levels measured in laboratory toxicity tests would be less than about 30 seconds for median current conditions, a period much less than that of the 24 hr or 96 hr acute toxicity tests. P. 739